Successful exploitation of wireless consumer products relies on highly integrated, low cost integrated circuits. Steady improvements in transistor performance and demand for higher levels of integration have led to the increased application of silicon technology for RF and wireless circuit applications. Indeed, cost effective silicon-based integrated circuits are now available for wireless personal communications systems at lower bit rates in the .about.1 GHz band.
Recent developments in broadband multimedia communications systems are based on wireless asynchronous transfer mode (ATM) transmission in the 5 GHz band. Although GaAs circuits remain several times more expensive than silicon circuits, the feasibility of using lower cost silicon based technology in this frequency band has been limited, due to significantly higher substrate and interconnect losses in silicon relative to GaAs. Historically, silicon technology has suffered from a lack of high Q inductors. More recently, improved inductor performance has been obtained using microstrip transmission line inductors.
Nevertheless, whether designing high frequency GaAs or silicon based circuits, for example, tuned low noise amplifier (LNA) and mixer circuits for wireless systems, simultaneous noise and impedance matching presents a challenge to improved performance. There is a trade-off in noise and input impedance matching, as discussed by K. K. Ko et al, "A comparative study on the various monolithic low noise amplifier circuit topologies for RF and microwave Applications" IEEE J. Solid State Circuits vol. 31, no. 8, August 1996, pp. 1220-1225. This trade-off is caused mostly by the fact that the transistor size is traditionally considered as a fixed design parameter, and a library of certain standards sizes are available. Thus, conventionally, a passive network is designed around a given transistor in order to achieve noise matching and/or impedance matching. The passive network itself contributes losses and degrades the noise figure, as discussed by F. McGrath et al, in "A 1.9 GHz GaAs Chip set for the personal handyphone system", IEEE Trans. MTT Vol. 43, pp. 1733-1744, 1995 and by A. Brunel, et al. in "A Downconverter for use in a dual mode AMPS/CDMA chip set", in Microwave J., pp. 20-42, February 1996.
The losses in the passive network increase as the network become more complicated, and a significant area of an integrated circuit may be taken up by the matching network. For example, in typical low noise amplifiers and GaAs mixer circuits discussed in the above mentioned references to Ko, McGrath and Brunel, either the noise figure or input impedance matching are sub-optimal, or the passive matching circuit is excessively complex, occupying a large semiconductor area.
It is well known that high frequency losses are particularly severe on semiconducting silicon substrates, relative to semi-insulating GaAs substrates. On the other hand, while passive components are less lossy on GaAs substrates, the present cost of GaAs circuits is at least a factor of two more expensive than silicon circuits. Consequently, if matching losses were reduced for silicon substrates to allow for design of high performance wireless circuits, these circuits could be fabricated in silicon with significant cost savings relative to similar GaAs circuits.